Programmable logic devices (PLDs) are a well-known type of integrated circuit that can be programmed to perform specified logic functions. One type of PLD, the field programmable gate array (FPGA), typically includes an array of programmable tiles. These programmable tiles can include, for example, input/output blocks (IOBs), configurable logic blocks (CLBs), dedicated random access memory blocks (BRAM), multipliers, digital signal processing blocks (DSPs), processors, clock managers, delay lock loops (DLLs), and so forth.
Each programmable tile typically includes both programmable interconnect and programmable logic. The programmable interconnect typically includes a large number of interconnect lines of varying lengths interconnected by programmable interconnect points (PIPs). The programmable logic implements the logic of a user design using programmable elements that can include, for example, function generators, registers, arithmetic logic, and so forth.
The programmable interconnect and programmable logic are typically programmed by loading a stream of configuration data into internal configuration memory cells that define how the programmable elements are configured. The configuration data can be read from memory (e.g., from an external PROM) or written into the FPGA by an external device. The collective states of the individual memory cells then determine the function of the FPGA.
FIG. 1 is a simplified illustration of an exemplary FPGA. The FPGA of FIG. 1 includes an array of configurable logic blocks (LBs 101a–101i) and programmable input/output blocks (I/Os 102a–102d). The LBs and I/O blocks are interconnected by a programmable interconnect structure that includes a large number of interconnect lines 103 interconnected by programmable interconnect points (PIPs 104, shown as small circles in FIG. 1). PIPs are often coupled into groups (e.g., group 105) that implement multiplexer circuits selecting one of several interconnect lines to provide a signal to a destination interconnect line or logic block. Some FPGAs also include additional logic blocks with special purposes (not shown), e.g., DLLs, RAM, and so forth.
Another type of PLD is the Complex Programmable Logic Device, or CPLD. A CPLD includes two or more “function blocks” connected together and to input/output (I/O) resources by an interconnect switch matrix. Each function block of the CPLD includes a two-level AND/OR structure similar to those used in Programmable Logic Arrays (PLAs) and Programmable Array Logic (PAL) devices. In some CPLDs, configuration data is stored on-chip in non-volatile memory. In other CPLDs, configuration data is stored on-chip in non-volatile memory, then downloaded to volatile memory as part of an initial configuration sequence.
For all of these programmable logic devices (PLDs), the functionality of the device is controlled by data bits provided to the device for that purpose. In many PLDs, the data bits are stored in volatile memory (e.g., static memory cells). Some PLDs include millions of these static memory cells. Therefore, it is clearly desirable to provide efficient circuit implementations that enable the reduction in size of static memory cells, to reduce the size of the overall PLD or other integrated circuit (IC) that includes the static memory cells.
One way of reducing the size of the static memory cells in an IC, as well as the size of the IC, is to use a semiconductor fabrication process that allows the use of smaller geometries in the various fabrication layers. As described in more detail below in conjunction with FIGS. 6 and 7, some fabrication processes use an imaging process known as “alternating aperture phase-shift masking”. Because these processes support the reduction of geometry sizes in PLDs and other ICs, it is clearly desirable to provide methods of implementing static memory cells that comply with the requirements of phase shift masks.